Throughout this specification the use of the word “Inventor” in singular form may be taken as reference to one (singular) or all (plural) inventors of the present invention. The inventor has identified the following related art. There are a number of ways of detecting particles in a region, such as a room, building, enclosure, or open space. Some methods involve sampling air from the region and passing the sampled air through a detection chamber, whereby particles are detected and estimation is made of the amount of smoke, for example, in the region of interest. Such an apparatus is exemplified in aspirated smoke detectors like VESDA® LaserPLUS™ smoke detectors sold by the applicant.
Other detectors are placed in the region of interest, and use a sensor to detect particles adjacent the sensor. An example of such a detector is a point detector, in which air passes between an emitter and a sensor, and the particles are detected directly in the region of interest.
In both cases if the particles do not enter a sampling point (of the aspirated detector) or pass between the sensor and emitter of the point detector, no particles will be detected. As many buildings employ air handling means for extracting air from a region, such as air-conditioning, there is no guarantee that suspended particles will be detected rather than pass out of the region via the air handling ducts. It can be very difficult to use the aforementioned methods of detecting particles in outdoor areas or very large indoor arenas where there may not be appropriate locations to place a point detector or a sample point and connecting tubing.
Other devices used to detect, for example, smoke include the detector disclosed in U.S. Pat. No. 3,924,252, (Duston) which uses a laser and a photodiode to detect light scattered from particles. This device uses a corner reflector to reflect the light back at the emitter. Duston requires a feedback circuit to detect whether the beam is emitted or blocked.
Another type of detector is known as a “Beam Detector”, which measures the attenuation of the intensity of a signal from a projected light source caused by smoke particles suspended in the projected light. These detectors, namely beam detectors and the detector disclosed in Duston, have relatively low sensitivity and are only capable of measuring the total attenuation within the illuminated region.
The above noted detectors may need to address a number of difficulties that are faced when attempting to detect particles by use of emitted radiation in a monitored area that may comprise, for example, indoor rooms, large indoor arenas and outdoor areas. Some of these difficulties comprise the following. The installation and commissioning of equipment to provide emitted radiation and means for detecting the emitted radiation and/or scattered radiation may be onerous. In particular, such equipment may be intrusive to the monitored environment and may require complex connections, for example, wiring or otherwise to supply control, communications and power to the equipment. Additionally, a number of technical personnel with particular skills may be required to install and/or commission the equipment. Once installed and/or commissioned such equipment may be susceptible to environmental conditions that form part of the monitored environment that contribute to drift, misalignment and the like to cause inaccuracies of measurement. Furthermore, there are environmental conditions and events unrelated to alarm conditions that may commonly occur in the monitored environment and may contribute to false alarms when detecting particles. It is desirable to detect particles in large rooms and areas and the physical distances that are involved may contribute to increasing the likelihood of the above noted environmental conditions and events having an effect on the efficiency of detecting particles and also, the distances involved relate to the path length to be travelled by radiation, which of itself requires equipment with high sensitivity and error tolerance.
Nuisance particles such as airborne dust, for example, may be present in the monitored environment and cause false alarms to be raised when there is no actual threat of fire outbreak. For instance, smoke particles are those generated as a result of thermal decomposition, such as in a smouldering fire, whereas nuisance particles may be generated without an underlying fire threat by, for example, mechanical or biological processes. Light scattering characteristics are related to particle size distribution; and there are many types of smoke and many types of nuisance particles and their particle size distributions often overlap. A light scattering method and apparatus using a light scattering cell for chemically identifying individual particles of matter or multiple particles of matter, such as found in aerosols, without collecting and chemically analysing the material is disclosed in U.S. Pat. No. 3,901,602 (Gravatt Jr). According to Gravatt, in the case of single particle analysis, plane-polarized light is impinged on the particle and the intensity of the light scattered into the plane of polarization over a specified angular range is measured. The intensity is related to the particle's coefficient of absorption and its size. In multiple particle analysis, the intensity of the light scattered into a plane perpendicular to the plane of polarization is also measured to determine the total number of particles of matter. This information may be used to normalize the intensity measurement of the first scattered light beam. A smoke detector is presented by Gravatt as an apparatus embodying the multiple particle analysis technique whereby fire-produced aerosols may be detected without interference from non-fire-produced aerosols of similar density.
Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Australia or elsewhere on or before the priority date of the disclosure and claims herein.